This invention relates to the technology of heat-sensitive recording apparatus using the thermal head. The invention particularly relates to a heat-sensitive recording apparatus that can produce high-quality image by reducing the K ratio unevenness which is the unevenness in image density due to voltage drop.
Heat-sensitive recording is used as a means of recording with various types of printer, plotter, facsimile, recorder, etc. Heat-sensitive recording has various advantages such as no need for wet development and ease of handling, so its application to image recording for medical diagnoses that require large and high-quality image as in CT, MRI and X-ray imaging is currently being studied.
As is well known, heat-sensitive recording uses a thermal head having heating elements arranged in one direction (the main scanning direction); with the heat-generating portion of the thermal head slightly pressed onto the heat-sensitive material, the two members are moved relative to each other in the auxiliary scanning direction which is normal to the main scanning direction and in accordance with the image to be recorded, the heating element for each pixel is supplied with energy to generate heat so that the recording layer of the heat-sensitive material is heated to record image.
In the thermal head, the heating elements arranged in the main scanning direction each have a heat-generating resistor and a switching element and are connected parallel between a power source and the ground. Each of the switching elements are turned on and off by a signal subjected to pulse-duration modulation in accordance with the image data value (density data value) for each pixel, so that the corresponding heat-generating resistor is energized for a time period equal to the pulse duration of the modulated signal; in this way, heat-sensitive image recording by individual heat-generating resistors is controlled.
In the thermal head, the power source and the heating elements are connected by various known methods such as the use of a flexible cable. In a configuration that allows for simplified fabrication and structure at reduced cost, a common electrode to all heating elements is formed to extend in the main scanning direction via a wiring pattern and both ends of the common electrode in the main scanning direction in which the heating elements are arranged are connected to the power source (this configuration is hereunder referred to as the xe2x80x9cboth end common supplyxe2x80x9d type).
When image recording is done by this thermal head, voltage drop proportional to the current flow through the thermal head occurs on account of various resistances including the internal resistance of the power source, wiring resistances such as the one of the power cable extending from the power source to the thermal head, and the pattern resistance or the resistance of the wiring pattern in the thermal head. Since the current flow through the thermal head varies with the image data, the amount of the voltage drop varies with the data for the image to be recorded. The amount of voltage drop that occurs in the thermal head also varies with the resistance of the wiring pattern as a function of the position in the main scanning direction.
As a result, the heat-sensitive recording apparatus using the thermal head undergoes variations in the supply voltage to each of the heating elements, causing a problem generally referred to as xe2x80x9cK ratio unevennessxe2x80x9d, which is the density difference that occurs in the actual recorded image despite the sameness of the image data.
To correct the K ratio unevenness, it has been attempted to calculate the average voltage drop per line or the voltage drop in each heating element and compensate for the loss of thermal energy due to the voltage drop. However, the K ratio unevenness cannot be adequately corrected by this method since it is incapable of dealing with the difference in the amount of voltage drop that occurs from the pattern resistance as a function of the position in the main scanning direction.
To deal with this situation, the assignee previously developed an algorithm for correcting the K ratio unevenness according to which the distribution of voltage drop in the wiring pattern as a function of the position in the main scanning direction was calculated on the basis of image data and on each heating element, multiplication by a correction factor as determined by the calculated distribution of voltage drop was performed; the assignee proposed this algorithm in a patent application (see Unexamined Published Japanese Patent Application (JPA) No. 291334/1998). According to this correcting algorithm, the K ratio unevenness can be effectively corrected in spite of the difference in the amount of voltage drop occurring as a function of the position in the main scanning direction and it is possible to produce image of better quality than in the prior art.
However, this technique sometimes fails to achieve satisfactory correction of the K ratio unevenness that occurs to the thermal head of the aforementioned xe2x80x9cboth end common supplyxe2x80x9d type and further improvements are to be made.
The present invention has been accomplished under these circumstances and has as an object providing a heat-sensitive recording apparatus that uses a thermal head of the xe2x80x9cboth end common supplyxe2x80x9d type using a common electrode to all heating elements, with it being connected to the power source such that drive power is supplied to it at both ends in the main scanning direction in which the heating elements are arranged, and which ensures that image deterioration due to K ratio unevenness is prevented in a more efficient and positive way to record image of better quality that is free from any density unevenness.
In order to attain the object described above, the present invention provides a heat-sensitive recording apparatus comprising: a thermal head having a plurality of heating elements arranged in a main scanning direction, a common electrode connected to all of the heating elements, and at least one connecting portion which connects the common electrode to a power source such that drive power is supplied at the heating elements located at both ends in the main scanning direction, wherein the common electrode is such that impedance from the at least one connecting portion to a heating element at one end in the main scanning direction is equal to impedance from the at least one connecting portion to a heating element at the other end in the main scanning direction; and an image processing section which, in accordance with image data, determines a current distribution within the common electrode for each of positions in the main scanning direction that correspond to the heating elements, the image processing section using the determined current distribution to determine amount of voltage drop due to resistance of the common electrode as a function of a position in the main scanning direction, and correcting resulting density unevenness in accordance with the image data based on both the amount of voltage drop due to the resistance of the common electrode as the function of the position in the main scanning direction and the amount of voltage drop due to resistances that are independent of the position in the main scanning direction.
Preferably, the at least one connecting portion has a plurality of the connecting portions and wherein the impedance from the power source to one connecting portion is equal to the impedance to another connecting portion.